In accordance with JP OS 138603/76 a workup comprises, among other things, the post-acetylation of free L-.alpha.-amino carboxylic acid and subsequent racemization of the post-acetylated mixture by addition of approximately equal mol amounts of acetic anhydride and heating a solution of the thus prepared mixture. The isolation of the relatively insoluble N-Acetyl-D,L-.alpha.-amino carboxylic acids can take place by acidification and suitable separation, for example, by filtration.
A more advantageous procedure is described in DE 37 02 689 which comprises the workup of the mother liquors from the enzymatic splitting. In this case, the L-.alpha.-amino carboxylic acid can be completely separated. As described in EP-A-O 175 840 the racemization of the free N-Acetyl-D(L)-.alpha.-amino carboxylic acid can proceed in a melt with catalytic amounts of acetic anhydride. By solution of the racemized N-Acetyl-D,L-.alpha.-amino carboxylic acids in aqueous sodium hydroxide there results a mixture which can again be subjected to acetylase splitting.
In accordance with the procedure of JP OS 138603/76 it is a particular disadvantage that for the therein described racemization of N-Acetyl-D(L)-.alpha.-amino carboxylic acids, approximately equal molar amounts of acetic anhydride are required.
There is a further substantial disadvantage in that the N-Acetyl-D,L-.alpha.-amino carboxylic acid is recovered through crystallization.
The known workup methods of DE 37 02 689 or EP-A-O 175 840 are likewise burdened with disadvantages in many ways. Thus, during the workup of the mother liquor by means of an ion exchanger, the N-Acetyl-D(L)-.alpha.-amino carboxylic acid is eluted as an aqueous solution which contains an amount of acetic acid corresponding to the sodium acetate. Because of the acid pH value of this solution, during the subsequent condensation there is a small amount of
hydrolysis to the free amino carboxylic acid which, during the subsequent racemization leads to a higher formation of acyldipeptide. Also the racemization described in EP-A 0 175 840 of free N-Acetyl-D(L)-.alpha.-amino carboxylic acids in the melt with catalytic amounts of acetic anhydride leads to the formation of acetyldipeptides.